In the lost sea, I dived down into the water,and there,I found the ruin of city with two treasure box nearby entrance. After entered the ruin, I can not open the octopus painted door at the end of passage. Please teach me how to open the door, and this place is really sunkened city or not. please teach me!!

Hmmmm, the obvious door by the chests is exit only, got that. The greek temple-like structure has an entrance under one corner - OK with that too. BUT - the third area has a square hole on top that leads into a room that seems to go nowhere, is this the third entrance you mentioned?

If so, how do you get into the city this way?
If not, where is the third entrance?

I did the whole of Collassium without using this 'third entrance' with no problem. So I'm a tad confused here, and it's Friday, and I want a beer, and I can't get any of this miserable lot at work here to come down the pub with me..... (mutter, mumble, **Expletive deleted**)